You can't

You get an alert about resending POST data if you go back to a page or refresh a page that was previously requested from the server by submitting form data via a (hidden) POST form.
Firefox can only make sure to get the same page by resending that POST form.
Firefox doesn't know what that form data means, so Firefox asks for confirmation before resending that form data as such an action can cause you to repeat an action and buy another item or post a message another time.
A way to prevent that pop-up is not to use the Back button, but to open links from a page that was requested from a server by sending a POST form in a new tab (window) with a middle-click or a Ctrl + left-click.
Then you can close that tab or window to go back.

You Can't? PPFT! I was just gone on vacation ( Aug 12 2011), got back and now I'm getting this, "To display this page, Firefox must send information that will repeat any action (such as a search or order confirmation) that was performed earlier" I was in a forum that I always go to (not at a merchandise website) and have NEVER had this message there or any where! I'm not about to load a bunch of tabs just so I can look back and forth. It seems Fire Fox has updated behind my back and now it's all messed up. I don't thing people would be complaining about it if it wasn't something unusual that just happened.

Is this possibly because of a change in FF6? I am developing a website, and before I upgraded to FF6 this was not a problem. I believe it is caused when I call:

window.location.reload(true);

If that page was originally entered by sending POST data via a form then reloading that page in any way will cause the confirmation dialog to appear.